Induction type time-delay relay



Sept. 20, 1949. H. J. cARLlN ET Al. 2,482,453

INDUCTION TYPE TIME DELAY RELAY Filed ost. 12. 1944 59 @L Fay, 7, l 1 a5 kL2 6 .E al

Ig WITNEssEs: INVENToRs /s/fw m. W www ATTORN EY Patented Sept. 20, 1949 INDUCTION TYPE TIME-DELAY RELAY Herbert J. Carlin, East Orange,v N.- J., and Bert V. Heard', Portland, Oreg., assignors to Westinghouse Electric Corporation, a corporation of Pennsylvania Application October 12, 1944, Serial No. 558,316

(Cl. F75-294) 12 Claims. l

This invention relates to electroresponsive instruments and it has particular relation to an induction type relay having a time delay in operation, which varies as ani'nverse function of the electrical quantity to which the relay responds.

The induction principle of operation is ernployed in many electrical instruments such as integrating and indicating measuring instruments and relays. Such instruments' may be designed ior response to'` various electrical quantities such as voltage; current and power. Since the invention is particularly suitable for time-delay electrical relays, the discussion will be directed to induction-type overcu'ri-'ent relays which are designed fior operationwith time delay.

Aninduction-type relay includes an electrocond-uctive armature which is mounted for rotation. lin order' to retard rotation of the armature,v damping means is. provided which may take the form of one or more permanent magnets which produce magnetic flux through the armature. The retard-ing force developed by such permarient. magnets varies in. accordance with the angular velocity ot rotation oi the armature.

InA order to obtain a force acting on they armature of an induction retay to produce rotation thereof., a. pair of alternating magnetic fluxes ci. and o2' are. applied'. toA the" armature. magnetic fluxes .are displaced both in space andi time phase from each other te provide a shifting magnetic field which urges the armature in a predetermined direction'. The angle of phase displacement between the magnetic fluxes` may be represented by the: reference Vcharacter 0. In conventional designs, the angle' 0 may have an optimum. value of the order of 4.151". If 7c designatos a suitable constant... the torque T applied to the` armature of the induction type.v relay may be represented bythe following expression:

Induction type relays are provided with magnetic structures or cores which provide paths of low reluctance for the lmagnetic fluxes o1 and oz. Ii the. relay is to: develop .a desirably large torque at large. values ot current, the magnetic core thereof should not saturate when energized by such currents. On the other hand, it is difcult to provide adequate ,torque inthe low current operating rangel of the relay unless the magnetic core of the relay is designed to saturate when energized by large.A currents. For these reasons, it is thepractice tnA employ ainagnetic core which saturates. whenlenergizedrby large currents which are within. the operating. range of. the relay. In a conventional induction-type overcurrent relay,

These CII 2 the ratio of maximum to minimum current in the operating range of the relay may be of the order of 20 to 1.

For many applications, it is desirable that a relay operate with inverse time delay. This means that the time of operation of the relay varies as an inverse function ci the magnitude of the electrical quantity to which the relay responds over the entire operating range of the relay.

Saturation of the magnetic core has an adverse effect von the inverse time .delay characteristics of the relay. saturation. restricts the increase oi either or both ot the magnetic fluxes e1 and 4e. Consequently, after saturation of the magnetic cores-the time'of .operation of the relay does not decrease at the desired rapid rate With an increase ,in energizing current..

In accordance with the invention, the effects of saturation of the magnetic core of a relay are compensated by ,suitahle changes in the' angle of phase displacement .0 'between .the two magnetic fluxes ci einem. By reference to the above-noted formula for torque, it will be observed that the effect oi sata 'on Iof the magnetic core of an induction relay is -terestrict the increase of magnetic fluxes c1 ggf. accordance with thek inver-ition,l this restriction eliset .or `compensated by suitably increasing thevalue .of the expression:

In this way, an induction type relay can be designed to havedesirable-inverse time delay characteristics over the entire operating range thereof. l

It is, therefore, an object or. the present inventionv to provide l,an ,electroresponsive instrument .having `iniprcyed tongue characteristics. K It is. a. further .objectief Yhe invention to provide a. time-,delay relay wherein eiects vof mag.- netic .saturation are. lsulist antially compensated.

It is a still punisher l:attirentoi the invention to provide an ndlltll t perelay -navi-ng an armature responsive to. the .torque developed by two phase-displaced, alternating magnetic fluxes wherein the phase between the magnetic fluxes is Maellsi il; yaorGlance with` the .energizationl or; the :compensate for the eiects .of saturation of the magnetic coreof the relay on the torque @enclosedthereby.`

Other -Obietscf .the ici/.ecticefwillte .apparent from. the. .Iollowngdeseription taken. inA conjunc- 'tion with .the `accorxnrgarnling. .drawing. in which:

Figure.- 1 is.aggraplncalgepresentationciopelatma curves for a .delay A Fig. 2 is a schematic representation, with parts 3 shown in plan elevation, of a relay system embodying the invention;

Fig. 3 is a plan view, with parts broken away, of an induction disk relay embodying certain features of the invention;

Fig. 4 is a vertical sectional View taken along the line IV-IV of Fig. 3;

Fig. 5 is a schematic development view showing the construction of the relay of Figs. 3 and 4 spread out in a plane and showing suitable connections for the relay; and

Figs. 6 and '1 are vector representations showing relations of currents in the relay of Figs. 3, 4 and 5.

Referring to the drawing, Fig. 1 shows the operating characteristics of conventional induction type inverse time-delay relays. In Fig. 1, ordinates represent time delays in operation of the relays and abscissae represent values of energizing current. The curve I represents a relay which is designed for inverse time-delay operation whereas the curve 3 represents a relay which is designed for what is known as very-inverse time-delay operation. It will be noted that for lower current values, the relays represented by the curves I and P 3 have time delays of operation which decrease rapidly with an increase in energizing current. However, for larger current values, the curves appear to approach asymptotes which are horizontal in Fig. 1. Consequently, in this latter range in increase in energizing current is not accompanied by a substantial decrease in the operating time of the relays. As previously explained, it is desirable that the inverse characteristics of the relays be available over the entire operating i ranges thereof.

The relay system illustrated in Fig. 2 is capable of providing improved inverse characteristics. In Fig. 2, a relay is represented which includes an electroconductive armature 5 in the form of a disk. This armature is mounted for rotation on a shaft 1 which carries a movable contact member 9. Rotation of the shaft 1 carries the movable contact 9 into and out of engagement with stationary contacts Closure of the contacts I| may be employed to complete an electrical circuit for any desired control or protective function. A spiral spring I3 is provided for biasing the shaft 1 in a predetermined direction. For the purpose of discussion, it will be assumed that the spring I3 biases the shaft to urge the movable contact member 9 away form the stationary contacts II.

Rotation of the armature 5 is retarded by suitable damping means which is illustrated in Fig. 2 as a permanent magnet I5. The armature 5 is positioned for rotation between the pole faces of the permanent magnet. As well understood in the art, the damping force provided by the permanent magnet I5 varies in accordance with the rate of rotation of the armature 5.

For applying an operating torque to the armature 5, windings I1, I9, and 2| are provided which, when energized by alternating current, direct alternating magnetic fluxes in a path cutting the armature 5. The magnetic fluxes of windings I1I9, and 2| correspond, respectively, to the previously-mentioned fluxes@ and Q52. The magnetic flux of windings I1 and |9 and the magnetic flux of the winding 2| are displaced in space with respect to the armature 5. If the magnetic flux of windings |1 and |9 also is displaced in time phase from the ux of the winding 2|, the magnetic fluxes develop a shifting magnetic field which produces a torque acting on the armature 5.

In order to provide magnetic paths of low reluctance for the magnetic flux produced by the various windings, a magnetic structure or core 23 is provided which includes a pair of pole pieces 25 and 21 for the windings I1 and I9 and a pole piece 29 for the winding 2|. These pole pieces are spaced to dene an air gap 3| within which the armature 5 is positioned for rotation. The construction and operation of the specific portions of Fig. 2 which thus far have been specifically referred to are well understood in the art and are disclosed, in part, in the Relay Handbook published in 1926 by the National Electric Light Association of New York city. Specic reference may be made to pages 104 to 125 of this Handbook wherein induction overcurrent relays operating with the time delay are discussed.

Continuing the discussion of Fig. 2, the windings I1, I9, and 2| may be connected for energization in any suitable manner. Let it be assumed that the relay of Fig. 2 is to be energized in accordance with the current owing in an alternating-current circuit represented by conductors LI and L2. Such energization may be derived from the circuit through a current transformer 33. The windings I1, I9 and 2|- and a resistor 35 are connected in series across the secondary of the current transformer 33. The resistor 35 will be discussed further below. It will be understood that the windings |1 and I9 are so connected that when the winding I1 directs magnetic flux downwardly in the pole piece 25, the winding |9 directs magnetic flux upwardly in the pole piece 21. The direction in which the magnetic eld produced by the windings urges the armature 5, may be reversed by reversing the connection of the terminals of the winding 2|.

Since the windings I1, I9 and 2|' are connected in series, the current flowing therethrough and the magnetic uxes produced by said current are in phase. To lag the air-gap magnetic flux p1 as produced by the winding 2| with respect to the air gap flux c2 as produced by the windings I1 and I9, a closed circuit lagging winding 31 is disposed about the pole piece 29. When the circuit of the lagging winding, 31, is closed, a torque is applied to the armature 5 which urges the movable contact member 9 towards the stationary contacts In considering the phase displacement between the magnetic fluxes p1 and p2 it may be noted that in an induction relay of conventional type the angle of phase displacement between the magnetic fluxes applied to the relay armature may have a value of the order of 45. However, to obtain a maximum of inverseness, the design of the relay shown in Fig. 2fmay be such that with low multiples of minimum trip current in the relay, the phase displacement of the fluxes 1 and c2 i-s considerably less than 45.

It will be recalled that saturation of the magnetic structure 23 is response to energization of the relay by large currents tends to restrict the increase of one or both magnetic fluxes (p1 and c2 and adversely affects the inverse characteristics of the time delay provided by the relay. To compensate for the effects of such saturation, an impedance i-s included in the closed circuit of the lagging winding 31 which varies, in magnitude as a function of the energization of the relay. Such an impedance is represented in Fig. 2 by a winding 39 having a closed magnetic core 4|. By saturating the magnetic core 4|, the re- 7 other. If the magnetic fluxes are displaced in time phase from each other by the angle and if K is a suit-able constant, then the torque T applied by :the magnetic uxes to the armature is represented by the well-known formula:

and 85 to a pair of secondary terminals of the .r

autotransformer which are capable of supplying substantial current at low voltage to these current-type windings. The windings are so arranged that when the windings on the pole pieces 5| and 55 direct magnetic ux downwardly therein, the windings associated with the pole pieces 53 rand 51 direct magnetic ilux upwardly therein.

In a somewhat similar manner, the windings 11 are connected through conductors 81 and 89 across secondary terminals of the autotransformer 19 which are capable of supplying relatively smaller currents at larger voltages to these low-current type windings. In order .to provide a desired phase displacement between current iiowing in the windings and current flowing in the windings 11, suitable impedances represented by resistors 9| and 93 are connected in series -with the windings 11. The currents I1 and I2 which low respectively through the windings 11 and 15 may be represented as shown in the vector diagram of Fig. 6, wherein the current I1 leads the current I2 by the angl-e 0. It will be understood that the windings 11 are so arranged that when the windings associated with the pole pieces 63 and `61 direct magnetic uX downwardly rtherein the windings associated with the pole pieces 65 and 69 direct magnetic iiux upwardly in the associated pole pieces. The structure of the relay illustrated in Figs. 3, 4 and 5 which thus yfar has been specically referred to is illustrated and described in greater detail in the copending application of Bert V. Hoard, Serial No. 456,901, iiled September 1, 1942, now Patent No. 2,379,905, issued July 10, 1945, and assigned to the same assignee to whom the .present application is assigned.

In the relay of Figs. 3, 4 and 5, saturation of one or both magnetic structures 41 and 49 affects the relationship between the magnetic uXes ci and e2 and the current which energizes the relay. In order to compensate for restrictions on increase in magnetic flux imposed by saturation, the relay is so designed that the magnetic structures 41 and 49 do not saturate equally. It is permissible with the structure illustrated in Figs. 3, 4 and 5 to so construct the magnetic structure 41 that it does not lsaturate even when energized by the maximum current for which the relay is designed. This is permissible for the reason that the magnetic structure 49 is designed to saturate and limit the maximum voltage appearing across the relay. Preferably, all of the pole-face members 6| and 13 have cross sections sufcient to prevent saturation thereof. This is desirable for the reason that each of the pole-face members carries both magnetic uxes. To obtain satura' tion of the magnetic structure 49, the pole pieces 63, 65, 61, and 69 or the yoke 1| or all of these parts may be provided with a cross section sufciently restricted to assure saturation thereof when the windings 11 are energized by current above a predetermined value but within the working range of the relay.

Saturation of the magnetic structure 49 decreases the reactive impedance offered to current I1 flowing through the windings 11. Consequently, this current as the magnetic structure 49 saturates, increases more rapidly than the current iiowing through the windings 15 and becomes more leading with respect to the current I2 owing through the windings 15. This is shown in Fig. 7 wherein the currents I1 and I2 are displaced in phase by an angle 0 which is substantially larger than the corresponding angle 0 of Fig. 6. For the reasons above set forth, the increase in the angle 0 may be proportioned to compensate for the restrictions in the increase of the flux ci imposed by saturation of the magnetic structure 49. By suitable proportioning, the operating torque applied to the armature 5 in Fig. 5 may be designed to vary in accordance with the square of the energizing current over substantially the entire range of operation of the relay.

As previously pointed out, rotation of a shaft may be retarded by associating a permanent magnet with an electroconductive armature mounted on the shaft. Although such damping may be provided for the relay of Figs. 3, 4 and 5, a somewhat diferent damping arrangement is illustrated in Fig. 5.

In Fig. 5, a pair of half-wave rectiers 95 and 91, together with resistors 99 and IUI, are connected in series between conductors 81 and 89. The rectiers may be of any suitable construction. Conveniently, they may be of the barrierlayer type, such as copper-oxide rectiers. The rectiiiers are so connected that they permit the ow of current therethrough in the same direction with respect to an intermediate terminal |03. In the specific embodiment of Fig. 5, the rectiiiers 95 and 91 permit the now of current only towards the terminal |83. Since the rectiiiers are oppositively directed with respect to the series circuit which includes the resistors 98 and |0|, current can not now directly through such series circuit.

The terminal |03 is connected by a conductor |85 to one terminal on each of the windings 11 associated with the pole pieces 65 and 61. The connections are such that when the conductor 81 is positive with respect to the conductor 89, current iiows from the conductor 81 through the resistor |0|, the rectier 91, the terminal |03, the conductor |95, the windings 11 associated with the pole pieces 65 and 63, and the resistor 9| to the conductor 89. On the other hand when the conductor 89 is positive with respect to the conductor 81, current iiows from the conductor 89 through the resistor 99, the rectier 95, the terminal |83, the conductor |85, the windings 11 associated with the pole pieces 61 and 69 and the resistor 93 to the conductor 81. Consequently, the windings 11 are energized by pulsating direct current and serve to produce a damping force acting on the armature I5. As previously explained, the damping force varies as the velocity of rotation of the armature 5.

In addition, the direct current owing through the windings 11 contributes to the saturation of the magnetic structure 49. Consequently, the proportions of the magnetic structure 49 should.

be such that saturation is obtained when the windings 'l1 are carrying predetermined values of both their direct and alternating current components within the desired range of energization of the relay.

Although the invention has been described with reference to certain specific embodiments thereof, numerous modifications thereof are possible. Therefore, the invention is to be restricted only by the appended claims as interpreted in View of the prior art.

We claim as our invention:

1. In an electroresponsive, alternating-current instrument having terminals, a rotor assembly, a stator assembly, means mounting said rotor assembly for movement relative to said stator assembly, damping means associated with said assemblies for retarding movement of said rotor assembly relative to said stator assembly, and means associated with said assemblies for producing, when energized through the terminals from an alternating current source, a plurality of phase-displaced alternating magnetic fluxes acting on said assemblies to produce relative movement therebetween, said alternating magnetic fluxes being effective for producing a force acting between said assemblies which varies in magnitude as a function of the phase displacement between said magnetic fluxes, and a rectifier unit connected for energization from the terminals for producing a direct-current output varying asa function of the input to the unit,

said assemblies including means responsive to' the direct-current output for changing the phase displacement between said magnetic fluxes.

2. In an electroresponsive, alternating-current instrument, a rotor assembly, a stator assembly, means mounting said rotor assembly for movement relative to said stator assembly, damping means associated with said assemblies for retarding movement of said rotor assembly relative to said stator assembly, means associated with said assemblies for producing, when energized, a plurality of phase-displaced alternating magnetic fluxes acting onsaid assemblies to produce relative movement therebetween, said alternating magnetic fluxes beingV effective for producing a force acting between said assemblies which varies in magnitude as a function of the phase displacement between said magnetic fluxes, one of said assemblies providing a magnetic path for one of said magnetic fluxes which is saturable independently of the path for the other of said magnetic fluxes, and direct-current means responsive to the energization producing the alternating magnetic fluxes for controlling the saturation of the'saturable magnetic path within the range ofoperation rof said instrument to change the phase displacement between said magnetic fluxes.

3, In an electroresponsive, alternating-current instrument, a rotor assembly, a stator assembly, means mounting said rotor assembly for movement relative to said stator assembly, damping means associated with said assemblies for retarding movement of said rotor assembly relative to said stator assembly, and means associated with said assemblies for producing, when energized, a plurality of phase-displaced alternating magnetic uxes acting on said assemblies to produce relative movement therebetween, said alternating magnetic fluxes being effective for producing a force acting between said assemblies which varies in magnitude as a function of the phase displacementk between said magnetic fluxes, and lagging means associated with the path of at least one of Said magnetic fluxes and adjustable for controlling the phase displacement between said magnetic fluxes, said lagging means comprising a direct-current rectifier connected for energization in accordance with a predetermined variable alternating quantity, and means responsive to the direct-current output of the rectifier for varying the adjustment of said lagging means to vary the phase displacement between said magnetic fluxes.

4. In an electroresponsive, alternating-current instrument, a rotor assembly, a stator assembly, means mounting said rotor assembly for movement relative to said stator assembly, damping means associated with said assemblies for retarding movement of said rotor assembly relative to said stator assembly, and means associated with said assemblies for producing, when energized, a plurality of phase-displaced .alternating magnetic fluxes acting on said assemblies to produce relative movement therebetween, said alternating magnetic fluxes being effective for producing a force acting between said yassemblies which varies in magnitude as a function of the phase displacement between said magnetic fluxes, lagging means associated with the path of at least one of said magnetic fluxes for controlling the phase displacement therebetween, said lagging means including an impedance having a magnetic core for controlling themagnitude of lagging by the lagging means, and direct-current means responsive to a predetermined variable quantity for saturating said 'magnetic core to vary the phase vdisplacement between said magnetic fluxes.

5. In an induction instrument responsive to an alternating electrical quantity, a magnetic structure having a plurality of pole pieces defining an air gap, an electro-conductive armature, means mounting said armature for rotation in said air gap, circuit means effective when energized in accordance with a predetermined alternating quantity for producing a plurality of phase-displaced alternating magnetic fluxes in said pole pieces and air gap, said magnetic fluxes acting on said armature to urge said armature in a predetermined direction relative to said magnetic structure, said circuit means having impedance values selected to provide a phase displacement between the magnetic fluxes for low values of energization of the instrument having a value differing from the value giving optimum torque urging the armature in said direction, damping means associated with said armature for retarding movement thereof, land means responsive to an increase inthe energization producing one of saidmagnetic fluxes for varying the impedance values and thereby varying the phase displacement of said magnetic iluxes to increase the torquelapplied to said armature by said magnetic fluxes. v A

6. In an electroresponsive induction instrument, an electroconductive armature, actuating means including first winding means effective, when energized, for producing a first alternating magnetic flux acting on said armature, and second winding means effective, when energized, for producing a second alternating magnetic flux acting on said' armature, said magnetic fluxes cooperating to develop a force acting on said armature which is a function of the phase displacement between said magnetic fluxes, means mounting said armature for movement relative to said actuating means, damping 'means' for retarding movement and magnetic means associated with said actuating means for establishing low reluctance paths for magnetic flux produced as a result of energization of said winding means, the phase displacement of the magnetic fluxes being dependent on the relative magnetic permeabilities of the low reluctance paths, said magnetic means including a portion associated with one of said magnetic fluxes, said portion being saturable within the range of operation of said instrument in ad- Vance of saturation of the portion of the magnetic means associated with the other of the magnetic fluxes for varying the phase relationship between said magnetic fluxes to increase the force acting on said armature.

'7. In an electroresponsive induction relay, an electroconductive armature, bearing means supporting said armature for rotation, and actuating means including a saturable magnetic structure responsive to a variable quantity for applying to said armature a pair of alternating magnetic fluxes which are space and phase displaced to develop a torque between said armature and said actuating means which is dependent on the magnitude of said magnetic fluxes and the phase displacement therebetween, said actuating means including the saturable magnetic structure being proportioned to provide magnetic fluxes having a phrase displacement below the value producing optimum torque for a,l predetermined range of operation of the relay, said saturable magnetic structure operating as a lagging control means for one of the magnetic fluxes energized in accordance with one of said magnetic fluxes, said magnetic structure being proportioned to saturate when said one magnetic flux exceeds a predetermined value to vary the phase displacement between said magnetic fluxes towards a value giving optimum torque.

8. In an electroresponsive inverse-time, induction relay, a magnetic structure having an air gap, said magnetic structure including first and second pole means having pole faces adjacent said air gap, rst winding means associated with said first pole means, said winding means being effective when energized by alternating current for directing a first magnetic'flux through said air gap, second winding means Vassociated with said second pole means, said second winding means being effective when energizedby alternating current for directing a second magnetic ux through said air gap, an electroconductive armature, means mounting said electro-conductive armature for rotation in said air gap, damping means associated with said armature, said magnetic fluxes being physically spaced to produce a torque Iactf ing on said armature Ywhich is dependent on the magnitudes of the magnetic fluxes and on the phase displacement thereof, said magnetic structure and the winding means having impedances selected to provide magnetic fluxes having a phase displacement which is other than the value producing optimum torque when one of said alternating currents is below a predetermined value, and means responsive to said one alternating current when it exceeds its said predetermined value for adjusting said phase displacement towards the value producing optimum torque.

9. In an electroresponsive inverse-time, induction relay, a magnetic structure having an air gap, said magnetic structure including first and second pole means having pole faces adjacent said air gap, first winding means associated with said rst pole means, said winding means being eifectivewhen energized by alternating currentv for directing a first magnetic flux through said air gap, second winding means associated with said second pole means, said second Winding means being effective when energized by alternating current for directing a second magnetic flux through said air gap, and electroconductive armature, means mounting said electroconductive armature for rotation in said air gap, damping means associated with said armature, said magnetic fluxes being physically spaced to produce a torque acting on Vsaid armature which is dependent on the magnitudes of the magnetic fluxes and on the phase displacement thereof, said mag- .netic structure and the winding means having impedances selected to provide magnetic fluxes having a phase displacement dependent on the relative permeabilities of the associated portions of said magnetic structure which is other than the value producing optimum torque when one "of said alternating currents is below a predetermined value, the portion of said magnetic structure associated with the leading one of said magnetic fluxes being designed to saturate in ad- ,vance of saturation of the portion of the magnetic structure associated with the remaining one of said magnetic fiuxes when said magnetic flux increases within the range of operation of the rey air gap, second winding means associated with said second pole means, said second winding means being effective when energized by alternating current for directing a second magnetic flux through said air gap, an electroconductive armature, means mounting said electroconductive armature for rotation in said air gap, damping means associated with said armature, said magnetic fluxes being physically spaced to produce a torque acting on said armature which is dependent on the magnitudes of the magnetic fluxes and on the phase displacement thereof, said magnetic structure and the winding means having imped'ances selected to provide magnetic fluxes having a phase displacement which is other than the value producing optimum torque when one of said alternating currents is below a predetermined value, and means responsive to said one alternating current when it exceeds its said predetermined value for adjusting said phase displacement towards the value producing optimum torque, said last-named means comprising a lagging coil associated with one of said magnetic fiuxes, and means responsive to said one altery nating current for varying the impedance of said directing a first magnetic flux through said air g-ap, second winding means associated with said second pole means, said second winding means being effective when energized by alternating current for directing a second magnetic flux through said air gap, an electroconductive armature, means mounting said electroconductive armature for rotation in said air gap, damping means associated with said armature, said magnetic fluxes being physically spaced to produce a torque acting on said armature which is dependent on the magnitudes of the magnetic fluxes and on the phase displacement thereof, said magnetic structure and the winding means having impedances selected to provide magnetic iiuixes .having la phase displacement which is other than the value producing optimum torque when one of said alternating currents is below a predetermined value, and means responsive to said one alternating current when it exceeds its said predetermined value for adjusting said phase displacement towards the value producing optimum torque, said lastnamed means comprising a lagging coil associated with one of said magnetic fluxes, an inductor, means connecting said inductor with said lagging coil in a series circuit, said inductor having a, saturable magnetic core, and means responsive to increase of said one alternating current above a, predetermined value for saturating said magnetic core to decrease the impedance of said series circuit.

12. In an electroresponsive induction instrument a stator assembly, an electroconductive armature, means mounting said armature for rotation relative to said stator assembly, alternating current responsive means for actuating said armature including a plurality of windings on said stator effective when energized in series from a source of alternating `current for supplying magnetic flux to said armature, and means for damping rotation of said armature, said damping means comprising a pair of half-wave rectiflers connected in series opposition, and means connecting a separate portion of said windings across each of said rectiiiers.

HERBERT J. CARLIN.

BERT V. HOARD.

REFERENCES CITED iy The following references are of record 1n the file of this patent:

UNITED STATES PATENTS 

